When a high electron mobility transistor (HEMT) device is actuated, due to certain characteristics of epitaxial material, such as the formation of an incomplete crystalline phase, a large number of impurities with negative charges can exist in an epitaxial layer which is located at the bottom of the device structure. At this time, if a high voltage is applied, these negative charges will be attracted to, and therefore move toward, the upper components of the device, affecting the actuation of the upper components. Similarly, it is also the material character of the epitaxial layer which provides a leakage path extending towards the substrate for the negative charges. However, no matter whether the negative charges in the device structure migrate towards the upper components, or whether they leak from the path extending towards the substrate, the actuation and safety of the components will be seriously impacted.
Therefore, development of a HEMT device which is capable of effectively removing negative charges at the bottom of the device is desirable.